Inactive window looks identical to focused — chrome should dim when the window loses key

[amanfromsolan]

Problem

When the Enso window isn't the key window, nothing in the app visually changes except the system drop shadow. Sidebar, header, tints, selection highlight — all render at full strength, so at a glance you can't tell whether the window has focus.

Native macOS apps dim their chrome when inactive: sidebar text and icons fade, selection turns grey, traffic lights lose their color. Enso should do the same.

Expected

When the window resigns key (and restores on becoming key):

• Sidebar/chrome fades — text, icons, selection highlight drop to a dimmed/disabled strength, or an overall fade over the chrome.
• Traffic lights grey out — they should show the native inactive treatment; worth verifying the custom titlebar handling (auto-hide alpha animation + the sidebar toggle pinned next to the zoom button in TerminalRootView.swift ~694) isn't interfering with AppKit's own inactive greying.

Notes

• The app's colors come from custom Theme.ink / Theme.text opacities rather than semantic NSColors (like labelColor/selectedContentBackgroundColor) that auto-dim in inactive windows — which is why nothing responds today. Either observe key state (NSWindow.didBecomeKey/didResignKeyNotification, or SwiftUI's controlActiveState) and drive a dim factor through the theme, or move toward semantic colors where it's cheap.
• TerminalRootView.swift already listens for didBecomeKeyNotification (~line 499) for other reasons, so there's a hook point nearby.
• The terminal content itself can stay as-is (terminals conventionally keep their colors); it's the surrounding chrome that needs to respond.